1. Field of the Invention
The present invention relates to the manufacturing of semiconductor devices. More particularly, the present invention relates to a wafer flat zone aligner for orienting wafers prior to their introduction into a facility in which the wafers are to be processed.
2. Description of the Related Art
Semiconductor products are being widely used in various industries, such as the electronics, computer and aerospace industries. Presently, such semiconductor products are under a rapid technical development to achieve high integration and high performance, i.e., so as to be capable of storing large amounts of data per unit area and/or so as to be capable of processing large amounts of data per unit time. The semiconductor products are fabricated by carrying out a plurality of very precise fabricating processes in facilities through which the overall semiconductor fabricating process progresses.
Data is stored or computations are performed in such semiconductor products by devices such as thin film transistors, thin film capacitors and thin film resistors. These semiconductor devices constitute minute electric circuits having a circuit pattern formed, based on a highly precise design rule of, for instance, about 0.10 microns, from a semiconductor thin film. In forming such accurate semiconductor thin film patterns, the above-mentioned semiconductor fabricating facilities perform very precise semiconductor thin film processes on a pure silicon wafer at each of numerous semiconductor chip areas formed on the wafer. Therefore, the wafer must be aligned with each semiconductor fabrication facility just before the semiconductor process is to be carried out in the facility on the numerous semiconductor chip areas formed on the wafer.
To this end, the wafer has a flat circumferential portion. This so-called xe2x80x9cflat zonexe2x80x9d of the wafer serves as a reference position in a process of aligning the wafer in preparation for introducing the wafer into the semiconductor fabrication facility in which the fabricating process is to be performed. Such a wafer having a flat zone is transferred to a wafer cassette after a first fabricating process is carried out, and is then transferred from the cassette to the next facility in which the subsequent semiconductor fabricating process is to be carried out. Thus, most semiconductor fabrication facilities perform a flat zone alignment process before the semiconductor fabricating process is carried out because the relative position of the wafer has likely changed from the designated position in the course of loading/unloading the wafer cassette or transferring the wafer.
The orienting of a wafer to a designated position is mainly carried out by a piece of equipment referred to as a flat zone aligner. The structure and operation of a typical flat zone aligner is described below.
A plurality of wafers are respectively disposed in slots of a wafer cassette having an open bottom. The wafer cassette is transferred to a designated position in the flat zone aligner. The flat zone aligner comprises two driving rollers made of a stainless material, and a guide roller made of quartz. The guide roller is disposed a little above the driving rollers. Once the cassette arrives at the designated position, the two driving rollers, and the guide roller are brought into contact with the wafers through the open bottom of the cassette.
The driving rollers are rotated by a motor while in contact with the circumference of the wafers. Any wafer having a flat zone that is not in alignment is contacted by the guide roller and the two driving rollers, and is rotated by the two driving rollers. Once the flat zone of the wafer confronts the guide roller, the wafer stops rotating and is thereby oriented in a position in which the wafer is aligned.
However, the wafer flat zone aligner described above causes several problems. For instance, the rollers can break or scratch the edge of the wafer. In addition, the aligner can produce particles when the rollers thereof collide with the wafers disposed in the slots of the wafer cassette.
Such problems can be caused by an inexact vertical displacement of the guide roller by an elevating mechanism of the wafer aligner. As mentioned above, the guide roller is made of quartz which is a hard but fairly brittle material. The pure silicon wafer is also hard but rather brittle. If the guide roller is raised too far by the elevating mechanism of the flat zone wafer aligner, the quartz guide roller and silicon wafers can collide with such an impact that the guide roller breaks, or the edge of the wafer cracks or breaks. In the case in which a fine crack is produced in the edge of the wafer, a mechanical stress concentration is produced at the crack when the wafer is thermally stressed during subsequent processing. As a result, the crack propagates from the edge to an active area of the wafer, thereby producing a defect in the semiconductor device(s).
The conventional wafer flat zone aligner can also scratch the wafers and produce particles as follows.
The flat zone aligner also includes a sensor unit for sensing whether there are wafers in the slots of the cassette. The sensor unit comprises confronting sensor plates that are inserted into the wafer cassette at positions at which each wafer, if disposed in a slot of the cassette, will lie between adjacent ones of the sensor plates. However, sometimes a wafer will butt up against an upper flat surface of one of the sensor plates as the sensor plates are inserted into the wafer cassette. Thus, the wafer will not come to lie in between adjacent ones of the sensor plates. Therefore, although this wafer is indeed present in a slot of the wafer cassette, the sensor unit issues an erroneous signal indicating that no wafer is present in the slot.
To avoid such problems, the wafer cassette is generally tilted in the wafer flat zone aligner by about 2 degrees relative to the horizontal. Accordingly, the wafers become tilted by about 2 degrees relative to the vertical so that as the sensor plates are inserted vertically into the cassette, the wafers present in the slots are self-guided, if necessary, to positions at which they become located between adjacent ones of the sensor plates.
The silicon wafers in such an inclined state are subsequently contacted and rotated by the driving rollers. Consequently, the wafers produce flutes in the circumference of the driving rollers due to friction. Due to the tilted state of the wafers, this flute extends over time as an oblique line relative to the outer circumference of the driving roller. Therefore, a portion of the wafer may be introduced into this flute during an alignment process and hence, will start to move along the flute, i.e., will start to move axially along the driving roller.
In this case, however, the other portion of the wafer is seated in the slot. Thus, the portion of the wafer contacting the driving roller is moved by the flute axially along the driving roller, but the other portion of the wafer seated in the slot can not move. Therefore, the wafer is driven into contact with the wafer slot and strongly collides with the wafer slot, thereby causing friction and noise, scratching the wafer, creating particles which may contaminate the wafer, and preventing the wafer from being rotated into the designated aligned position.
Accordingly, an object of the present invention is to provide a wafer flat zone aligner that substantially obviates one or more of the limitations and disadvantages of the prior art.
More specifically, one object of the present invention is to provide a wafer flat zone aligner that prevents wafers from being strongly forced against the walls of the cassette, that define the slots in which the wafers are seated, as the wafers are being rotated into designated aligned positions.
To achieve this object, the wafer flat zone aligner of the present invention comprises a wafer rotating roller that includes a shaft portion for rotating the wafers using friction, and a plurality of parallel spaced apart annular members protruding from the shaft portion. Each wafer seated in the cassette is inserted between adjacent ones of the annular members into contact with the shaft portion of the roller. As the wafers are being rotated by the shaft portion, the wafers are constrained from moving in the axial direction of the roller by the annular members. Thus, the wafers will not be forced against and bind to the walls of the cassette.
In addition, the shaft portion of the wafer rotating roller may have an outer jacket of silicone. The silicone will provide just enough friction with the wafers to overcome any force causing the wafers to adhere to the walls of the cassette.
Another object of the present invention is to provide a wafer flat zone aligner that when used will not cause an edge of a wafer to crack and will not prematurely degrade the guide roller thereof.
To achieve this object, the guide roller of the wafer aligner is made of an elastic material, such as polyethylene (PE) or polyetheretherketone (PEEK), that produces little friction with the wafers. Thus, neither the wafers nor the wafer guide roller itself will be damaged when the wafer guide roller is moved into contact with the wafers.